The following description pertains to the application of the present invention in a steam turbine. The statements made analogously also apply to gas turbines.
Steam turbines are used mainly as powerplant turbines for generating electricity and as industrial turbines for driving generators, pumps, fans and compressors. The steam turbine is a heat engine with rotating rotors, in which the enthalpy gradient of the continuously flowing steam is converted into mechanical energy in one or more stages.
The blading of the rotating rotor of the turbine shall convert the enthalpy of the steam into kinetic energy possibly in a lossless manner and transmit the forces occurring in the process to the shaft and the housing of the turbine. The steam now flows from a space having a higher pressure through a nozzle into a space being under a lower pressure. The greater the pressure difference, the greater is the velocity of the steam attained. After the discharge from the nozzle, the steam reaches the curved profile of the first rotor blade stage, the so-called regulating stage. Subsequently, the deflection takes place in the stationary guide vane stage for a subsequent flow through the next rotor blade stage again. Depending on the design and the size of the turbine, the process is repeated several times. The profile length of the rotor blades and guide vanes increases in the direction of flow. As a result, the space flown through increases, as a consequence of which the pressure and the temperature of the steam decrease. Large turbines are divided into a high-pressure part, a medium-pressure part and a low-pressure part.
The profile of each blade is a compromise between fluidic, strength-related, vibration-related and economic requirements. The blade profiles are available with mostly geometrically graduated chord lengths. The blades in a turbine are subject to many different loads and stresses. To guarantee a long operating time and to avoid damage, the blades must be designed and dimensioned correspondingly for safety. A rotor blade must have, for example, a sufficient strength in order to absorb the load caused by the centrifugal forces occurring as well as by the bending due to the torque to be transmitted. Additional load factors are the temperature at the inlet, which reaches up to 530° C., and the erosion corrosion occurring on the profile inlet sides due to the moisture content of the steam in the low-pressure range.
In addition to the stress caused by centrifugal forces, temperature and erosion corrosion, the rotor blades are subject to stress due to vibration. Vibration is induced in the rotor blades by the flowing steam in conjunction with other acting forces. The stress due to vibration leads, in the long term, to a change in the microstructure of the blade material. Incipient cracks of submicroscopic size are formed at first in the near-surface area, and they merge over time. After the damaging phase of the merging of the cracks, an incipient technical crack is finally formed, which extends at right angles to the highest principal direct stress and induces a considerable excessive increase in stress at the tips of the cracks. If the crack is not recognized or the blade is not replaced, fatigue fracture will occur at the end of the process. Damage due to stress due to vibration is among the most frequent causes of damage in material engineering, partly because the actual stress groups are unknown and partly because no complete theory can be set up as a consequence of the large number of material engineering influential factors.
Among other things, the following solutions are used to damp the vibrations of the rotor blades of steam turbines.
A wire extending circularly in holes in the profile area damps the vibrations in larger end-stage blades in the low-pressure range of the turbine.
In rotor blades, which are loaded by a low circumferential velocity only, a shrouding is riveted in sections by means of rivet pins to the profile end of the blades installed in the turbine rotor. This design was frequently used in older turbines. The strength of the riveted connection is not sufficient in modern turbines with high circumferential velocities. The riveted design is ruled out here.
Cover plate rotor blades, which combine good strength properties with high efficiencies, are now used almost exclusively in the high-pressure and medium-pressure areas of turbines. The blade and the piece of shrouding (cover plate) belonging to it form one unit in this design. The cover plates of the individual rotor blades form a ring after their installation in the turbine rotor. The vibration is damped in them at the contact surfaces between the individual blades. The drawback of the low strength of the riveted connection is thus avoided.
However, the design of the rotor blades provided with cover plates has the following weak points. It is not always possible in practice to install the rotor blades without clearance in relation to one another because of the different tolerances of each rotor blade in a stage with, e.g., 100 rotor blades. Another reason is the strong centrifugal forces, which act on every individual rotor blade in the operating state of the turbine. The centrifugal forces cause the blades to be somewhat offset to the outside. Since each rotor blade forms a wedge with its foot and cover plate surfaces, a gap is formed at the cover plate surfaces between the individual rotor blades due to the described outward settling of the blades. The vibrations are no longer damped as described because of the gap formation.
Several prior-art solutions are available for avoiding the described drawback caused by the gap formation between the cover plates of the rotor blades.
A plane groove each, in which a circular wire is introduced, is turned in the two plane faces of the cover plates after the installation of the rotor blades in the turbine rotor. The blades are connected with one another by the wire, and the vibrations are damped. The drawback of this solution is that a sufficient cover plate height must be available to install the wire. Heavy weight of the cover plates leads to a reduction of the possible speed of rotation of the turbine because of the events that are to be taken into account in the calculation of the strength.
In a second design, the cover plates are manufactured with a slight angular twisting in relation to the blade foot. After their installation in the turbine rotor, the rotor blades are under a certain torsional stress, which compensates the gap formation and guarantees damping of the vibrations as a result. However, this solution is expensive because of the manufacturing technology and difficult to design.
Furthermore, the rotor blades must have a certain minimum length for their use in order to make it possible to generate a torsional stress in the first place. In the longer term, the stress decreases due to wear on the contact surfaces and material fatigue. Vibration damping is no longer present thereafter.